Default clients to 2026 discovery

[leehack]

Summary

• default MCP clients to start with server/discover negotiation for the 2026 RC path
• preserve legacy initialize fallback for method-not-found and legacy Streamable HTTP pre-initialize 400 responses
• update tests, examples, and the CLI conformance harness for default discovery, explicit legacy opt-out, and 2026 removed ping behavior

Validation

• dart format lib/src/client/client.dart test/client/client_test.dart test/client/client_elicitation_defaults_test.dart test/client/client_tool_validation_test.dart test/client/streamable_https_test.dart test/elicitation_test.dart test/lifecycle_test.dart test/mcp_2026_07_28_test.dart example/client_stdio.dart packages/mcp_dart_cli/lib/src/conformance_runner.dart
• dart analyze
• dart test
• dart analyze (packages/mcp_dart_cli)
• dart test (packages/mcp_dart_cli)
• dart run bin/mcp_dart.dart conformance --suite all --json (packages/mcp_dart_cli)
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.48%. Comparing base (c22ba3f) to head (52f0c9c).

Files with missing lines	Patch %	Lines
lib/src/client/client.dart	91.66%	1 Missing ⚠️

Additional details and impacted files

@@                                Coverage Diff                                @@
##           spec/stateless-legacy-task-notification-guard     #249      +/-   ##
=================================================================================
- Coverage                                          88.48%   88.48%   -0.01%     
=================================================================================
  Files                                                 47       47              
  Lines                                               9345     9353       +8     
=================================================================================
+ Hits                                                8269     8276       +7     
- Misses                                              1076     1077       +1     

Flag	Coverage Δ
unittests	88.48% <91.66%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[leehack]

Closing this draft micro PR because the RC spec work has been consolidated into a smaller reviewable stack. This change is preserved in #271 (type/schema validation); the consolidated tip is tree-identical to the previous micro-stack tip.